This reverts commit ce766c73.

 * Update django-two-factor-auth to current release
 * Allow phunnumber-field 3.x or 4.x while django-two-factor-auth
   still crunch https://github.com/Bouke/django-two-factor-auth/pull/326
 * Run poetry update
   * Update django-image-cropping and easy_thumbnails for DJango 3.0 support
     (cf. !61 and #75)

Closes #117.

It calls black and isort on the source tree.

This finalises the migration to the black code style by applying double-quotes.

The following separate testenvs are defined (with their commands to run):

  tox              Run test suite
  tox -e lint      Run flake8 tests (including mypy and bandit)
  tox -e security  Run safety to check for vulnerable dependencies
  tox -e docs      Run sphinx to build documentation
  tox -e build     Run poetry to build sdist and bdist_wheel

Poetry 1.0.0 has been released